I think it's generally better to use `inherits` than `is`. From the `inherits` docs:

> inherits is nearly always equivalent to is, both for S4 and non-S4 objects, and is somewhat faster. The non-equivalence applies to classes that have conditional superclasses, with a non-trivial test= in the relation (not common and discouraged): for these, is tests for the relation but inherits by definition ignores conditional inheritance for S4 objects.

This might sound like a minor point, but I have a vague memory of `is` being a performance bottleneck in some shiny tag stuff we worked on a while back.